1. Field of the Invention
The present invention relates to an image forming apparatus that detects a phase of a photosensitive drum, and forms an image on a sheet using an electrophotographic method.
2. Description of the Related Art
As an electrophotographic image forming apparatus, there is generally known a system in which a surface of a photosensitive drum is uniformly charged, and the charged surface of the photosensitive drum is exposed by an exposure device to thereby form a latent image. In this electrophotographic image forming apparatus, an image is formed according to a potential of the latent image formed on the photosensitive drum.
Therefore, if the photosensitive drum has variation in characteristics, it is not possible to form an image which is uniform within the surface of the photosensitive drum, which causes degradation of image quality. Causes of degradation of image quality include non-uniform density due to variation in charging potential on the surface of the photosensitive drum, and position deviation of the image due to eccentricity of the photosensitive drum.
Conventionally, to correct the above-mentioned non-uniform density and position deviation, there has been proposed a method of correcting non-uniform density and position deviation by controlling a light amount and an exposure position by an exposure device according to a position of the photosensitive drum in a rotational direction (hereinafter referred to as the “drum phase”).
On the other hand, in controlling the exposure device according to the drum phase as mentioned above, it is required to detect a phase of the photosensitive drum. As a method of detecting a phase of the photosensitive drum, there is known a method of detecting a phase by providing a home position sensor (hereinafter referred to as the “HP sensor”) for the photosensitive drum, and detecting a reference position of the photosensitive drum by the HP sensor.
For example, there has been disclosed a method of detecting a phase of the photosensitive drum by providing two or more HP sensors for the photosensitive drum (see e.g. Japanese Patent Laid-Open Publication No. 2006-215269).
However, in the above-mentioned method of detecting a phase of the photosensitive drum by using the HP sensor, phase detection cannot be performed before the reference position passes over the HP sensor after the photosensitive drum has started to rotate, and hence it takes some time to detect a phase, which reduces productivity.
Although in the method disclosed in Japanese Patent Laid-Open Publication No. 2006-215269, it is considered that by providing two or more HP sensors, it is possible to reduce the phase detection time compared with the arrangement having one HP sensor. However, there is a problem that the use of a plurality of HP sensors results in an increase in costs.